To prepare recycled pulp from waste paper at low cost, inks and adhesive contaminants contained in the waste paper must be efficiently removed from fibers. Typically, inks are removed by washing or flotation after treating fibers with a mechanical ink-detachment machine to promote stripping of the inks because the inks stick to the fibers and ash. As to adhesive contaminants, it is known that they are most effectively removed by treating a low-consistency pulp slurry using a screen having a narrow slit width.
On the other hand, the recent diversification of prints has led to the frequent use of hard-to remove inks such as UV inks and varnish as well as the frequent inclusion of adhesive contaminants derived from adhesive labels or adhesive tapes, which tends to degrade the quality of recycled pulp. To compensate for such decline in quality, production processes of recycled pulp tend to involve heavier facilities, which invites problems such as yield losses and increase in electric power cost.
In view of the decline in the quality of raw materials, it was difficult to prepare recycled pulp at low cost while maintaining its quality at the same time. Attempts were made to solve such problems by applying fiber fractionation to production processes of recycled pulp and separately treating the resulting fractions to incorporate them into paper.
Japanese Patent No. 2986525 proposes a method for obtaining recycled pulp by fractionating fibers into a long fiber fraction and a short fiber fraction using an inward flow screen, treating the long fiber fraction by a fine screening process and the short fiber fraction by flotation, then combining both fractions and treating them with a cleaner followed by dehydration. This method is said to be advantageous in that the facility can be downsized by subjecting the long fiber fraction to no flotation. However, this method had the disadvantages that it requires a large screen to fractionate fibers at low consistency of approximately 1% because it uses an inward flow screen liable to be clogged with raw materials and that if one intends to remove inks during the flotation process of the short fiber fraction rich in inks, the flotation yield decreases, but if one intends to maintain a good yield, ink removal is not promoted and brightness decreases because the inks stick to fines and ash.
JPA2004-131892 proposes a method comprising defibering unsorted waste paper and treating it through a coarse screen followed by flotation, and then fractionating the resulting suspension using a screen having a slit width of 0.05 to 0.12 mm. This method required a large flotation facility to treat a large volume because fractionation is preceded by flotation. For example, 4 tons/hour or more of solids must be treated to obtain 100 BD tons/day of recycled pulp, which means that a large flotation facility capable of treating a slurry at 400 tons/hour or more is required in the case of flotation at a consistency of approximately 1% by weight known to ensure a good flotation efficiency.
JPA2006-316400 proposes a method comprising fractionating a pulp into a fine fraction and a coarse fraction using a pressure sorter, screw press or hydrocyclone, and separately subjecting both fractions to flotation under different conditions. However, this method had the disadvantages that when a screw press is used, fractionation conditions are difficult to control, and that when a hydrocyclone is used, separation takes place at low consistency so that the separation efficiency is poor despite a large facility involving high energy consumption. This method also had the disadvantage that when a pressure sorter is used, the yield decreases and the cost increases because both fractions are subjected to flotation most responsible for yield losses in the production process of recycled pulp. Further, this method had the disadvantage that the ink separation efficiency is too low to avoid yield losses if one intends to attain good brightness because the fine fraction is subjected to flotation at higher consistency than the coarse fraction without any means for separating inks sticking to fines and ash in the fine fraction from these fines and ash, similarly to patent document 1.